Whereas the conventional resistor such as the one disclosed by the laid-open patent Hei 4-102302 had been known, the conventional resistor and its manufacturing method are reviewed by referring attached drawing FIG. 23 showing a cross-section of the conventional resistor. In FIG. 23, 1 is an insulation substrate, 2 is a first surface electrode layer disposed on the left and right ends of insulation substrate 1, and 3 is a resistor layer disposed on first surface electrode layer 2 partly overlapping with first surface resistor layer 2. 4 is a first protection layer disposed to cover the entire surface of resistor layer 3, and 5s are trimming grooves disposed on resistor layer 3 and first protection layer 4, and said trimming grooves are cut when the resistance of resistor 3 has to be trimmed. 6 is a second protection layer disposed only on the surface of first protection layer 4, and 7 is a second surface electrode layer extended over the entire width of insulation substrate 1 on the surface of first surface electrode layer 2. 8s are side-electrode layers disposed on the both sides of insulation substrate 1, 9 and 10 are nickel-plated and solder-plated layers respectively disposed on second surface electrode layer 7 and side-electrode layer 8.
At this time, solder-plated layer 10 is disposed at a height lower than the height of said second protection layer 6. That is, second protection layer 6 is disposed at a highest height on the conventional resistor.
The manufacturing processes of conventional resistor having the above-shown construction are now explained below by referring the attached drawings.
FIGS. 24 and 25 show manufacturing processes of the conven-tional resistor. As shown in FIG. 24(a), first surface electrode layer 2 is formed by applying a coating on the left and right ends of insulation substrate 1. Then, as shown in FIG. 24(b), resistor layer 3 is formed by applying a coating on the both ends of insulation substrate 1 overlapping partly with resistor layer 3. Then, in next, as shown in FIG. 24(c), after coating first protection layer 4 covering the entire surface of resistor layer 3, trimming grooves 5 are formed on resistor layer 3 and first protection layer 4 by applying a laser-trimming method in order to set the resistance of resistor layer 3 within a predetermined range.
Then, as shown in FIG. 25(a), a second protection layer 6 is disposed on the surface of first protection layer 4, and as shown in FIG. 25(b), second surface electrode layer 7 is disposed on first surface electrode layer 2 extending over the entire width of insulation substrate 1. Then, as shown in FIG. 25(c), side-electrode layers 8 electrically connecting first surface electrode layer 2 to second surface electrode layer 7 are disposed on the both sides of insulating substrate 1. After applying a nickel-plating and a solder plating on the surfaces of second surface electrode layer 7 and side-electrode layer 8 as shown by a cross-section shown in FIG. 23, the resistor of conventional construction provided with nickel plated layer 9 and solder plated layer 10 are obtained.
However, with the resistor of conventional construction and manufacturing processes, since second protection layer 6 is deposited at a height higher than solder-plated layer 10, the resistor can be mounted on a circuit board (not shown) by utlizing only the back surface of insulation substrate 1. Therefore, there should be a definite disadvantage excluding the automatic resistor mounting machine from the mounting work of circuit boards because of the lack of ability discriminating the top or the bottom of resistors.